The invention relates to a hydraulic radial piston engine.
Radial piston engines, preferably externally mounted, slow-running radial piston engines that generate high levels of torque can be designed as dual-range engines, in which all the torque-generating pistons can be acted upon by hydraulic fluid from a pump, hence the hydraulic engine is operated at maximum absorption volume, at which it will produce a maximum amount of torque, while the rpm is decreased; on the other hand, it is also possible to operate the hydraulic engine with only a portion of the torque-generating pistons, in which a portion of the torque-generating pistons remain connected to the flow of hydraulic fluid from the pump, while another portion of the torque-generating pistons are connected to the flow of hydraulic fluid back to the pump or to the hydraulic fluid reservoir. In this operating condition the hydraulic engine generates a lower level of torque at the same level of pressure, however it can generate a higher rpm while the capacity of the pump remains the same.
In German Patent Specification 19 05 455, a hydrostatic radial piston engine is disclosed, which is equipped with a valve that connects the flow of hydraulic fluid from the pump to all the torque-generating pistons; when this valve is reversed, a portion of the torque-generating pistons become connected to the return flow of hydraulic fluid, hence the hydraulic engine is operating with only a portion of the torque-producing pistons, causing it to generate a higher rpm and lower torque. The valve is switched over by hydraulic fluid acting on a piston of the valve, shifting it against the force of a spring, or if the piston is pre-stressed against the spring, the valve is switched over in that the hydraulic fluid that acts on the piston is connected to a hydraulic fluid reservoir, hence the spring forces the piston back to its initial position. With transmissions of this type, a portion of the torque-generating pistons is suddenly connected to either the hydraulic fluid intake or the hydraulic fluid return flow. Since the speed of the drive is not able to adjust to this new situation as quickly, a severe shifting shock is created
In DE 196 37 570 A1 a hydrostatic-mechanical wheel drive is disclosed, in which a hydraulic radial piston engine drives an inner, central gear of a planetary gear set. The hydraulic piston engine can be slowed via a brake, which is hydraulically lifted and is closed via spring resistance, and always becomes engaged when the hydraulic fluid in the fluid intake drops below a certain pressure level. In order to keep the brake from becoming engaged if the pressure level in the hydraulic fluid intake drops briefly below the predefined pressure level, the spring must force the hydraulic fluid against a throttle. If a hydraulic engine is used to drive a mobile construction vehicle, such as an excavator, as is disclosed in DE 196 37 570 A1, critical safety situations may result if an automatic valve control system is used to connect some or all of the torque-generating pistons to the hydraulic fluid intake. If the hydraulic drive system is equipped with an automatic shifting device, which, when a previously established level of pressure in the intake of hydraulic fluid from the hydraulic pump to the hydraulic engine is exceeded, shifts the displacement switchover valve such that all of the torque-generating pistons are connected to the hydraulic fluid intake, and, if the hydraulic fluid intake falls below this previously established level of pressure, the shifting device reverses the displacement switchover valve so that only some of the torque-generating pistons are connected to the hydraulic fluid intake, then when the vehicle is traveling uphill, when the level of pressure in the hydraulic fluid intake is above the established pressure level, the hydraulic engine will operate at its maximum absorption volume. The displacement switchover valve is thus reversed, such that all of the torque-generating pistons are connected to the hydraulic fluid intake. If the driver reduces the flow of hydraulic-fluid from the pump to the hydraulic engine, in order to come to a stop on an incline, then the level of pressure in the hydraulic fluid intake to the engine will drop, so that the displacement switchover valve is reversed, and only a portion of the torque-generating pistons are connected to the hydraulic fluid intake. Because the brake is opened by force of pressure and closed by force of a spring, when the brake is closed, which occurs automatically when the pressure in the intake to the hydraulic engine drops below a predefined pressure level, then hydraulic fluid must be forced out of the brake actuating device. In order to prevent the brake from becoming engaged automatically with only a very brief drop in pressure, the hydraulic fluid in the brake-actuating device must be forced against a throttle position, causing the brake to engage at a delay. However, because switching the hydraulic fluid intake from all of the torque-generating pistons to only a portion of the torque-generating pistons creates a situation in the displacement switchover valve in which all of the torque-generating pistons are directly connected to the hydraulic fluid return flow, but the brake has not become engaged, the construction vehicle will roll down the hill until the brake is engaged, or until the connection is closed again. This can lead to critical safety situations. If the construction vehicle comprising an automatic shifting device, in the case of which below a certain pressure level only a portion of the torque-generating pistons are connected to the hydraulic fluid intake, moves downhill and if the driver of the construction vehicle reduces the flow of hydraulic fluid to the hydraulic engine in order to brake the vehicle, then the hydraulic engine is able to generate a maximum braking moment with only a portion of the torque-generating pistons with the pressure level of the brake valve or pressure limiting valve. This can result in a critical safety situation.
The object of the present invention is to create a hydraulic radial piston engine in which the shifting shock that occurs when the hydraulic fluid intake is switched from all of the torque-generating pistons to a portion of the torque-generating pistons, or when the hydraulic fluid intake is switched from a portion of the torque-generating pistons to all of the torque-generating pistons, is reduced, and in which no critical safety situations can occur in the operation of the hydraulic radial piston engine.
According to the invention, the valve that connects the torque-generating pistons to the hydraulic fluid intake is actuated via hydraulic fluid, the inward and outward flow of which is throttled, hence the throttled valve is forced against a spring tension, or the throttled valve is forced by the spring tension against the hydraulic fluid that is flowing off. Because the valve controls the inflow of hydraulic fluid to the pistons, and is itself reversed by throttled hydraulic fluid flowing in or out, a sealing point must be positioned between the hydraulic fluid intake to the hydraulic engine or the pistons and the actuating chamber for the hydraulic fluid of the valve, since otherwise hydraulic fluid could drain out of the hydraulic fluid intake to the hydraulic engine or to the pistons, via a gap in the valve actuating chamber, and could thus negatively affect the throttled actuation of the valve. The throttling point in the intake and the return for actuating the valve that connects the torque-generating pistons to the hydraulic fluid supply can have a defined value, or can be adjustable based upon operating parameters, such as the temperature of the hydraulic fluid or rpm. Preferably, the piston of the valve that connects the torque-generating pistons with the hydraulic fluid intake is designed such that when the valve is reversed from a first position to a second position, the cross-section, which connects or divides the two portions of torque-generating pistons, changes almost evenly over the shifting path of the piston, so that no sudden change in the cross-section of the flow of hydraulic fluid will occur; hence, undesirable shifting pressure is prevented. This is preferably realized in that the sides of the connecting grooves in the valve piston are designed as tapered surfaces, or are equipped with grooves, which are shaped such that the cross-section changes almost evenly with the shifting of the piston. The valve piston must be designed such that the cross-section changes nearly evenly with a shift from all of the torque-generating pistons to a portion of the torque-generating pistons, and with a shift from a portion of the torque-generating pistons to all of the torque-generating pistons. The valve that connects the torque-generating pistons to the hydraulic fluid intake is preferably positioned coaxially in a hydraulic fluid intake that supplies hydraulic fluid to a piston carrier. Thus the valve is housed in a structural chamber of the hydraulic radial piston engine that otherwise would be unused. It is also possible, however, for the valve that connects the torque-generating pistons with the hydraulic fluid intake to be positioned outside of the radial piston engine in order to reverse the displacement of the hydraulic radial piston engine. The valve that connects the torque-generating pistons with the hydraulic fluid intake is preferably reversed via spring tension such that when the flow of hydraulic fluid to the hydraulic radial piston engine drops below a certain pressure level, all of the torque-generating pistons become connected to the hydraulic fluid intake. This serves to ensure that, if there is a defect in the hydraulic fluid intake, the hydraulic radial piston engine can be operated with all of its torque-generating pistons, and thus at its maximum torque for the purpose of reversing the valve. Preferably, the hydraulic radial piston engine is connected via a planetary gear set to a wheel drive for a tracked vehicle, preferably an excavator, however it can also be directly connected to the driven shaft. This serves to ensure that with a compact wheel drive, sufficient torque will be generated. Because the piston carrier of the hydraulic engine is connected to a brake, the braking torque is intensified via the planet stage. To ensure that no critical safety situations can occur, a valve is positioned in front of the hydraulic fluid intake to the valve that is used to reverse the displacement of the radial piston engine; this additional valve is held in an initial position by force of spring tension, until the pressure in the hydraulic fluid intake to the hydraulic radial piston engine exceeds a certain level. When this valve is in its initial position, the hydraulic fluid intake for reversing the valve designed to connect the torque-generating pistons to the supply of hydraulic fluid is interrupted. This serves to ensure that below a certain pressure level in the hydraulic fluid intake to the hydraulic engine, all of the torque-generating pistons are constantly connected to the hydraulic fluid intake. This ensures that in the case of an intentional braking process when the vehicle is traveling downhill the hydraulic engine will brake with all of the torque-generating pistons against the brake valve or the pressure control valve. If the vehicle is to be stopped while driving uphill, and if in driving up the hill all of the torque-generating pistons are connected to the supply of hydraulic fluid in order to generate maximum torque, then even when the vehicle is held in a stationary position on a hill, all of the torque-generating pistons will remain connected to the hydraulic fluid intake, without causing an undesired shift, thus keeping the vehicle stationary on the hill until the brake has been engaged. In this, the level of pressure required to actuate the valve to reverse the displacement switchover valve is above the level of pressure required to lift the brake. Because the valve for reversing the valve that connects the torque-generating pistons with the hydraulic fluid supply is positioned in the hydraulic fluid intake, any shifting while the vehicle is stopped on the hill is prevented, independent of a manual or automatic shift command by the driver. Because this valve is positioned in front of the valve that connects the torque-generating pistons to the hydraulic fluid intake, this valve is preferably equipped with the throttle points through which the hydraulic fluid flows, in order to reverse the valve that connects the torque-generating pistons with the hydraulic fluid intake. If the hydraulic radial piston engine has a brake, then the pressure that lifts the hydraulic brake is also preferably used to reverse the valve that is positioned in front of the valve that connects the torque-generating pistons to the hydraulic fluid intake. This offers the advantage that no additional valve that constantly stems the flow of hydraulic fluid is necessary.
Because the flow of hydraulic fluid that controls the valve that connects the torque-generating pistons to the hydraulic fluid intake is throttled in its inward and its outward flow, and because it is guaranteed via a sealing point that this throttled inward and outward flow will not be negatively affected by any external hydraulic fluid, any undesirable shifting shock is prevented with an upward or downward shift. Because the valve that connects the torque-generating pistons to the hydraulic fluid intake is preceded by a valve, which, when the flow of hydraulic fluid to the hydraulic engine drops below a certain pressure level, interrupts the flow of hydraulic fluid that controls the valve that connects the torque-generating pistons to the hydraulic fluid supply, it can be ensured that, when the flow of hydraulic fluid to the hydraulic engine drops below a certain pressure level, all of the torque-generating pistons will be connected to the flow of hydraulic fluid to the hydraulic engine. In this manner, critical safety conditions are prevented.